The Truth is Out There

I'll start by saying the one thing we're really pretty sure of: dark matter is real. We've known for a very long time that the matter we can see in our telescopes -- stars, galaxies, gas, dust -- doesn't have enough gravitational pull to explain the motions of the cosmos. There are some verygoodexplanations of dark matter and its evidence on the web out there already, but in brief, given how fast stars and galaxies are moving (stars moving in galaxies, galaxies moving in clusters), the matter we can see isn't enough to hold them all together. The first evidence of this came out in the 1930s, and since then, astronomers have hypothesized that some mysterious new component of matter that we can't see -- dubbed dark matter -- is pervading and surrounding galaxies and clusters and keeping everything from flying off into space.

If this was the only evidence, it might be reasonable to suggest that it's not a new form of matter, but rather an altered law of gravity that explains the inconsistency. But it turns out that evidence for dark matter being a fundamentally new kind of matter pops up virtually everywhere we look -- from the way light bends around massive objects, to the history of galaxy formation, to the chemical make-up of the early universe. Some of the strongest evidence for dark matter is found in the aftermath of collisions of galaxy clusters, since these cosmic train wrecks can effectively separate dark matter from stars and gas.

So we know dark matter is out there, but we don't know what it is. We think it's probably some kind of new elementary particle, and the leading theories all suggest that it should have some interaction with light and/or ordinary matter (i.e., particles contained in the Standard Model of particle physics), so over the last few decades the physics community has put a lot of effort into finding a way to detect those interactions. There are basically three approaches:

Direct detection: If dark matter is a new elementary particle that interacts mainly via gravity and only very weakly via any other force, dark matter particles should be passing through the Earth all the time, and, very occasionally, you'd expect one to bump into something. Direct detection experiments look for that collision, called "nuclear recoil" (because you're looking for the movement of the atomic nucleus, not the electrons). Basically they put a box full of some target material (in the case of the CDMS experiment, that's silicon or germanium) in a heavily shielded lab deep underground where virtually no standard model particles can get in. Then, very sensitive detectors watch for one of the target nuclei to be bumped. If the scientists can rule out other explanations for the bump (like radioactive decay of the material around the target sending in neutrons, for instance), and if the recoil energy is what they expect dark matter to produce, then they have a dark matter event candidate.

Indirect detection: In many of the models of dark matter, the dark matter particle is its own antiparticle, which means that if two dark matter particles collide precisely enough, they annihilate. In theory, this produces standard model particles that we can see. If that's correct, then one way to find dark matter particles is to look at where dark matter is densely concentrated (like in the Galactic Center) and see if there are gamma rays or high-energy particles being produced in a way that ordinary astrophysics can't explain. There are other ways dark matter particle physics could be probed with direct detection, like if the particle decays or has other (non-annihilating) interactions with itself or other matter, but annihilation is the most common thing to look for. One of the reasons we think annihilation happens is because it leads to a natural way to explain the production of dark matter in the early universe -- the idea being that dark matter was annihilating and being produced all the time in the beginning when the universe was very dense, and it was only when expansion allowed dark matter particles to interact less frequently that they were able to exist in a more or less stable way for long periods of time.

Collider production: If two dark matter particles can annihilate to make standard model particles, then you should be able to reverse the process and make dark matter particles by colliding standard model particles at high energies. This is the idea behind the search for dark matter at colliders such as the LHC. A dark matter particle produced in a collider would pass right through the surrounding detectors without leaving a mark, so the way we'd see it would be to look for "missing energy." You add up all the energy of all the particles you do detect in the collision aftermath, compare it to the total energy you put in, and see if the missing energy is consistent with what a dark matter particle would spirit away.

Different ways to detect dark matter (DM) particle interactions with standard model (SM) particles. Image found on the MPIK website, originally produced by Jonathan Feng. "Thermal freeze-out" is what happens when the dark matter is no longer dense enough to annihilate all the time due to the expansion of the early universe.

So, have we found it yet?

There's been a lot of hype. A few weeks ago, the team behind an experiment called AMS-02, a cosmic ray detector that hangs off the side of the International Space Station, made an announcement that they found a signal "consistent" with dark matter, but that it was "not yet sufficiently conclusive to rule out other explanations." They held a press conference and released a short publication summarizing the work. As I pointed out in a blog post for IoP's Physics Focus, the tone of the announcement, and the especially media hype that followed, went far beyond what was really justified by the results. What they actually saw was an excess of positrons over what would be expected from standard astrophysical processes. The excess might have arisen from dark matter annihilation. But it could also have come from something else. Like pulsars, which are known to accelerate particles and which could certainly produce a positron excess like the one seen by AMS. I go into more detail on this in my Physics Focus blog post, but the gist is that while the AMS signal is intriguing, it's really difficult to pin it on dark matter with any degree of certainty.

But that didn't keep the media from running away with the idea. Here's a sampling of the kinds of headlines I saw in response to the AMS result:

Being excited about the prospect of a big discovery is fair, but overhyping it doesn't help anyone. Especially because only a couple of weeks later, another experiment, called CDMS, also claimed a possible detection of dark matter, and news articles said pretty similar things, sometimes without even referencing AMS:

Actually, the CDMS result got quite a bit less press, which was surprising to me. Pretty much any way you look at it, it's a much more direct result, if (as I'll explain) fantastically confusing.

Deep dark secrets

CDMS (or, specifically, CDMS-II) is an underground dark matter direct detection experiment. It's located in an old iron mine in Minnesota and it consists of super-cooled targets of silicon and germanium surrounded by sensitive detectors that can measure the positions and energies of any movements they see in their target nuclei. They expect to see, as a background, electron recoils from a variety of processes, and they can distinguish these from recoils of nuclei by looking at the way bumped electrons would ionize the target material. There are a number of ways they slice up the data to take out the electron background, but they also expect a tiny number of neutrons to get into the detector (either from space or from radioactive decay more locally) and bump into their target nuclei, and these would look exactly like dark matter collisions. The only way to deal with those is to estimate the number they expect from neutrons, and get excited if they see way more than that.

The dark matter candidate events found by the CDMS-II experiment. Plot from presentation by Kevin McCarthy at the APS meeting. The full presentation can be found here and the paper is here. I was alerted to this plot by this tweet.

In the end, CDMS found three candidate events. In the plot above, they're labelled Candidate 1, 2 and 3. (I hope the CDMS folk actually named the candidate events, in the style of the IceCube collaboration, who found two extragalactic neutrinos and called them Bert and Ernie.) The collaboration claims that the chance of these events actually being dark matter -- as opposed to misidentified background or random chance -- is 99.81%. That corresponds to what we call a 3-sigma result, which, by particle physics convention, is officially "evidence" but not officially a "detection." For comparison, the Higgs Boson discovery was deemed a true discovery when it reached 5-sigma.

Mixed signals

Obviously, a result at 99.81% confidence, while maybe not quite a detection, is intriguing. And, due to CDMS's ability to distinguish backgrounds, I would say it's far more intriguing than the AMS result as far as dark matter implications are concerned. But there are a number of very good reasons the physics community is staying cautious on this one. The biggest reason is that the simplest model of dark matter that could explain the CDMS result has already been ruled out by other experiments. There are lots of detectors in the direct detection game right now, and at the moment, many of them seem to be giving us very conflicting information. There have been detections -- tentative or otherwise -- claimed by four different experiments now, if CDMS is included. The others are DAMA/LIBRA, CoGeNT, and CRESST -- and, actually, a previous signal was claimed by CDMS but has since been considered more likely to be background). All these results could be signs of a dark matter particle -- specifically, a weakly interacting massive particle (or WIMP), but it's difficult to find a way to make them agree with one other. They all seem to find particles with different masses and interaction rates. Even worse, combining the constraints from other experiments, such as XENON and EDELWEISS, and even previous results from CDMS, seem to rule out all the claimed detections.

Constraints and hints from direct detection experiments. The horizontal axis is the mass of the dark matter particle and the vertical axis measures its interaction with standard model nuclei. Filled regions indicate signals interpreted as dark matter; lines indicate upper limits. Everything to the upper right of a line is ruled out to 90% confidence by that experiment. The lines are, roughly from left to right: XENON100 (dark dash-dotted green), XENON10 (light dash-dotted green), CDMS II Ge (dark and light dashed red), EDELWEISS (orange diamonds), and CDMS II Si (dark blue solid and black dotted). The asterisk is the best-fit point for CDMS's candidate events. This plot and more details can be found in arXiv:1304.4279 by the CDMS Collaboration.

AMS wasn't discussed in the CDMS paper, but I should point out that the best candidate dark matter model for the AMS result and the CDMS dark matter candidates do not agree either. It's a little difficult to compare them directly, because one is looking at dark matter annihilation and the other at dark matter interactions with nuclei, but the inferred particle masses are very different. To explain the AMS result, the dark matter particle would need a mass in the TeV (trillion electron-volt) range, whereas CDMS needs a particle with a mass a thousand times lighter. (Even though it's technically energy, an electron-volt is used as a measure of mass for fundamental particles, via E=mc2. GeV is a billion electron-volts and TeV is a trillion. For comparison, a proton is 0.938 GeV.)

AMS? CDMS? Total mess?

In the astro/physics community, the response to the result from CDMS has been mixed.

It's really not clear what we should make of all these conflicting results, and it's even less clear how to reconcile them. It could be that several of the experiments have just made mistakes or been misinterpreted, and with more data and more careful analysis we'll find out which signals were actually background events or random fluctuations. Or, it could be that dark matter is way more complicated than we realized. For instance, maybe it interacts differently with protons than with neutrons, or maybe there's more than one kind of dark matter particle, or maybe we've made an error with our assumptions about how dark matter is distributed in our galaxy, and fixing that will alleviate some of the tensions in the data. A few papers posted recently have also argued that the CDMS analysis of the XENON 100 constraint made it out to be more constraining than it is, so the CDMS result is maybe not entirely ruled out by XENON 100. But even that wouldn't explain all the other signals, and the results still don't easily agree.

As usual in science, the only thing to do now is to get more data. The business of dark matter detection is still in a fairly early stage -- as the detectors take more data and become more sophisticated, hopefully these signals and limits will start to make more sense. And of course we will keep looking in other places. The LHC is starting to place interesting limits on the dark matter parameter space, and even beyond AMS, efforts at indirect detection is also giving us some intriguing signals that may or may not have anything to do with dark matter. Some of us (e.g. me) are also looking toward the early universe to see if we can find hints for dark matter's effects on the first stars and galaxies.

Meanwhile, the preprint archive is happily aglow with new theory papers trying to piece this all together. It really is an exciting time; sometimes it's fun to have no idea what's going on.